An acoustic wave device equipped with comb electrodes is known as an acoustic wave device utilizing an acoustic wave. The comb electrodes are formed by an IDT (Interdigital Transducer) on a surface of a piezoelectric substrate, and are supplied with electric power, so that an acoustic wave can be excited. The acoustic wave device is widely used in various circuits processing radio signals in a frequency band ranging from 45 MHz to 2 GHz, such as a bandpass filter for transmission, a bandpass filter for reception, and an antenna duplexer. Recently, an acoustic wave device using a piezoelectric thin-film resonator has been employed in practical use. An exemplary piezoelectric thin-film resonator is an FBAR (Film Bulk Acoustic Resonator) in which electrodes are provided on opposite surfaces of a piezoelectric substrate so as to face each other. The piezoelectric substrate vibrates in the thickness direction. The device using the piezoelectric thin-film resonator exhibits good characteristics at higher frequencies and is used in a frequency range of 1 GHz to 10 GHz.
Recently, it has been required to downsize signal processing devices and to thus reduce acoustic wave devices used therein. Particularly, module devices are frequency employed in portable electronic devices such as cellular phones, and surface mountable compact devices are demanded. The acoustic wave devices need a cavity above a functioning portion that is essential to maintain desired characteristics. Such a functioning portion is comb electrodes for the surface acoustic wave device, and is a region in which upper and lower electrodes face each other across a piezoelectric thin film for the piezoelectric thin-film resonator. Such a cavity is defined in the wafer state in order to satisfy demands for downsizing. The acoustic wave device thus formed may be used as a module component by flip-chip mounting it on an electronic circuit board and sealing it with resin for transfer mold. The electronic circuit board may be a board having wiring or interconnections for forming an electronic circuit, such as a printed circuit board or a module board.
FIG. 1 is a cross-sectional view of an acoustic wave device mounted on an electronic circuit board. Referring to FIG. 1, an acoustic wave device 24 is flip-chip mounted on interconnection or wiring lines 18 of an electronic circuit board 26 by protrusion electrodes 22 formed on the acoustic wave device 24 and made of solder. A gap between the acoustic wave device 24 and the electronic circuit board 26 is defined as standoff 32. The height of the standoff 32 is defined by the protrusion electrodes 22. The acoustic wave device 24 is hermetically sealed with resin 30 for transfer mold.
Various ways to control the height of the standoff 32 have been proposed. Japanese Patent Application Publication Nos. 10-13012 (Document 1) and 9-213743 (Document 2) propose to use a spacer separate from the protrusion electrodes 22 between the acoustic wave device 24 and the electronic circuit board 26. The spacer functions to control the height of the standoff 32.
The technique of using a solder ball including an embedded resin core has been developed. The solder balls are used to flip-chip mount the acoustic wave device 24 on the electronic circuit board 26. The solder balls function as a spacer to control the height of the standoff 32.
Japanese Patent Application Publication No. 2004-296497 (Document 3) discloses a technique of covering side surfaces of the protrusion electrodes 22 with a resin layer. FIG. 2 is a cross-sectional view of an acoustic wave device (first related art) having the protrusion electrodes 22 made of solder in which the side surfaces thereof are covered with a resin layer. The protrusion electrodes 22 of solder are attached to the acoustic wave device 24. The side surfaces of the protrusion electrodes 22 are covered with resin layers 36. When the acoustic wave device of the first conventional art is flip-chip mounted on the electronic circuit board 26, the protrusion electrodes 22 that defines the height of the standoff 32 are covered with the resin layers 36, which function as a spacer and controls the height of the standoff 32.